WHY DOES EARTH SLOWS DOWN

As earth rotates , the Moon's gravity causes the oceans to seem to rise and fall . The sun also does this , but not as much there is a little bit of friction between the tides and the turnings earth causing the rotation to slow down just a little . As earth slows it let's the moon creep away.
Earth's graceful 24hours rotation rate is one of the traits that makes our planet so friendly to life allowing most parts of earth to stay a nice comfortable temperature as they are bathed in sunlight during day and darkness at night.
Each planet in a solar system has it's own unique rotation rate. Tiny Mercury . Sizzling closest to the sun takes earth days to turn around just once.